Flooding is the most damaging natural disaster worldwide, and the flood-vulnerable population is expected to grow in coming decades (1). Flood risks will likely increase because of both climate change (1) and shifting land uses, such as filling of wetlands and expansion of impervious surfaces, that lead to more rapid precipitation runoff into rivers. In the United States, annual river flood losses continue to rise (2), punctuated by major events in the Midwest (1993, $30 billion in total costs; 2008, $15 billion) and California's Central Valley (1995 and 1997; $4 billion each event) (3). Meanwhile, pressure to develop new housing in flood-prone areas near rivers (floodplains) continues (4), even as levee-system maintenance is chronically underfunded (5).
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